Photometer for measuring exposures and densities



Feb. 13, 1940; P. R. TARR 2,190,553

PHoToMETER Foa MEASURING Exposus AND DENsI'rIEs Filed May 14, 1938 Patented Feb. 13, 1940 A l UNITED STATES PATENT oFFicE PHOTOMETER FOR MEASURING EX- POSURES AND DENSITIES i Philip R. TamCuyahoga Falls, Ohio Application May 14, 1938, Serial No. 208,000 Claims. (Cl. 88--23) 'I'his invention relates to photometry'and par- It is essential, therefore, that the sensitized ticularly to-a photometer constructed with parpaper or other material be so chosen that its ticular reference toits use in projection printexposure scale matches that of the negative,

lng whichis commonly known as enlarging otherwise either over-contrasting or flat contrastl The general object of the invention is to provide less prints will be obtained.

a visual photometer particularly adapted for A further object of the present invention,

measuring and evaluating both exposure time and therefore, is to overcome the diiiculty of matchmeasuring and evaluating the density or contrast ing the paper and negative scales by providing scale 0f a, photographic negative, means to measure the maximum and minimum l0 There are numerous obstacles or difficulties in light intensities of the projected images to thereprojection printing such, for instance, as the seby permit a paper having the proper exposure lection of the proper grade of sensitized material scale to be selected for use with that negative. to suit the negative and the determination of the A still further object is to provide a photomproper time of exposure, which varies with the eter which may be also used in testing the exgrade or type of sensitized material. Such difiiposure scale of a sensitized paper and may be 15 culties are well known to those acquainted with used in determining the speed factor of the paper the art of projection printing, and a further oband further, to provide a photometer which can ject of the invention, therefore, is to provide a be used for evaluating exposure and images on photometer which will overcome to a large degree the ground glass of a camera where, as in these diiiiculties. microscopy of certain kinds, the spectral distri- 20 There are Wide differences in sensitized mabution of light is similar to the spectral sensibility terials for projection printing (these materials of the eye, or in Contact printing where the negabeing usually sensitized paper but may be sensitive is placed in contact with the paper.

tized glass or other material of-llke character). My invention is illustrated in the accompany- AdEach manufacturer produces special materials ing drawing wherein: 25

for certain uses, each of which materials is dif- Fig. 1 is a top plan view of a photometer conferent from the others as to its sensitivity to the structed in accordance with my invention. action 'of light during exposure, exposure being `Fig. 2 is a vertical section on .the line 22 of defined` as the intensity of light impinged upon Fig. 1. v a0 the surface of the sensitized material multiplied A Fig. 3 is an elevation of the absorption screen 30 by the time it is allowed to act. Since each phoattened out. tographic negative varies in density and each Fig. 4 is a perspective view of the comparison sensitized material for projection printing is block and the slide upon which it is mounted. different in its requirements as to exposure, it Referring to the drawing, I0 designates gencan be readily seen that unless accurate means erally the circular casing of the instrument which 35 is at hand for measuring the light intensity fallat one point is formed with a rectangular chaming upon the sensitized media, said sensitized ber II extending radially from the casing IIl and media will have to be exposed by means of guess constituting a housing lfor a comparison block I2,

,and trial. This is a costly practice producing which will be later described in detail.l This hous- 40 questionable results due to the factv that it is ing is closed on three sides and on the fourth side 40 impractical to try more than a very few of the by a wall I3 having an aperture I4 opening into many available materials with each negative. the main casing I0. The top wall of the housing The density or contrastfscale of a photographic II has a horizontal portion I5 and inward of negative refers to the relative amount of lightthis horizontal portion there is an upwardlyand absorbing material present in various parts of the centrally inclined hood I6 within which is dis- 45 negative image. In reference to projecting printposed a viewing lens Il which is focused upon the ing, however, it is a common practice to evaluate adjacent corner of the comparison block I2. The the density scale of a negative in terms of exhorizontal portion I5 of the top Wall is formed posure scale or the relative intensities of light with an aperture I8. This provides for the entransmitted through the various parts of the trance of a beam of light from an outside source, 50 negative while as to the sensitive material upon this beam being designated X and this being a which the projection print is made, its exposure beam of light, the intensity of which is to be scale refers to the relative exposures necessary measured. The side wall, as before stated, has toproduce the faintest perceptible-image and the an aperture I4. These apertures I8 and I4 are blackest or darkest image possible. so disposed that the beam X will impin'ge against 55 the bottom of the casing I0.

the upper surface of the comparison block I2 at the upper inner corner thereof while a beam Y passing through the aperture I4 will impinge against the upper end of the vertical surface of the comparison block I2.

Just inward of the opening i4 there is provided a diffusing screen i9 of ground glass or transluscent glass an'd a screen of neutral tint.

Within the casing I0 adjacent the screen I9 there is disposed a small electric bulb 20 mounted in a clamp 2|, which may be of any Suitable construction, which clamp is adjustable toward or from the'screen I9 by means of a slot 22 through which passes an adjusting screw or set screw 23. Any other means for adjusting this lamp to Ward or from the comparison block I2 may be provided. The comparison block I2 lis also mounted upon a slide 24 which is longitudinally slotted and which is held in adjusted position by the screw 25. The light comparison block has its upper face shielded from reflection as by a covering 26 and the inner face of this block is also shielded by a covering 21. These coverings 26 and 21 are cut away at 28 and 29, so as to provide two light-reflecting surfaces at right angles to each other. Thelight Y impinges Wdirectly upon the surface 29 while the light ray X impinges directly upon the surface 28.

The central portion of the casing I0 is provided with the upstanding tubular bearing 39. Operating over the upper open portion of the casing IU is a dial 3| having a knob 32 whereby lit may be rotated and supported upon a rotatable shaft 33 extending down through the tubular bearing 39, the dial and knob being held upon this shaft by means of a set screw 34. The lower end of the shaft 33 is engaged by a screw 35 having a relatively large head 36 disposed within a recess on Carried by the dial and extending downward into the casing ID is a continously variable absorption screen 31 which has a relatively high degree of transparency at one point on the screen and a relatively high degree of opacity at what may be termed the other end of the screen. The light controlling or absorption screen 31 is arranged so as to control the light from the comparison lamp 20 through the diffusing screen I9, this ray of light from the com parison lamp in turn illuminating the surface 29 of comparison block I2 continuously as the dial 3| is rotated from one end of the screen 31 to the other and from full intensity at one end of the screen to the lowest visible intensity at the other end of the screen while the light intensity to be measured illuminates the surface 28 of the com parisonlolock, both surfaces being visible along the axis 38 and being magnied by the lens I1. The dial 3| is provided with graduations conforming to some predetermined plan as to the light values transmitted to the comparison block I2, these graduations being read in connection with an index 39 carried on the wall of casing I0. Where the two surfaces 28 and 29 are equally illuminated, a null point is reached, this null point being obtained by rotating the dial 3| until the comparison surfaces are equally illuminated, reference being then made to the index point 39, reading the light value on the scale d, these values having previously been obtained by calibration,

The dial 3| can be calibrated by using a light of known Value to produce the beam X and varying the light intensity by moving the light source (not shown) axially along the beam X so as to produce varying intensities of light on the comparison surface 29 according to the inverse square law of light. At each desired change of intensity on the surface 29, the dial 3| may be rotated so that absorption screen 31 adjusts the light on surfaces 28 and 29 until they come to the same intensity when viewed along the optical axis 38, thereafter marking these units at the index point 39 so as to form the scale 40.

Referring again to the comparison block I2, this block is of white material and the faces 26 and 21 of the block are blackened out so that the semi-circular surfaces 28 and 29 are white and at right angles to each other, there being a sharply dened dividing line 4I at the intersection of these two surfaces.

In order to obtain accuracy in a photometer of this character, the voltage impressed upon the comparison standard lamp 20 through the leads 42 is to be adjusted so that the color of the filament in the lamp 29 produces a spectrum as similar as possible to that of the light to be measured. Where these conditions cannot be met, the eye may be somewhat assisted in making unity or "nullwadjustments of light intensity on the sur faces 2S and 29 by viewing these surfaces through a blue or green-colored filter 43 on the upper side of the lens I3, or in any other convenient location along the spectral axis 38. While the use of a filter does not completely correct color difference, it does assist if the observer uses the unbalance or shadow location method of reading the photometer.

do not wish to be limited to the particular character' of the screen 31 which, as before stated,

should vary very gradually in density from one 'r end of the screen to the other. There are various methods of making a screen of this character.

One is the provision of ruled lines running longitudinally along a transparent support as, for instance, aV strip of Celluloid, the lines progressively becoming broader from one end to the other, thus reducing light transmission through the screen by means of changing the ratio of transparent to opaque area progressively and very gradually from one end of the screen 31 to the other.

It is known that the maximum sensitivity of photographic printing papers is in the blue-green end of the spectrum while the maximum sensi`v tivity of the eye is in the yellow-orange region. However, through the choice of a proper comparison lamp, the ratio of blue-green to yelloworange light can be made alike in the enlarging and photometric apparatus, and it so happens when using ordinary incandescent lamps at their normal voltage that a change of intensity through an absorption screen (holding the filament voltage constant), in the region of the eyes maximum sensibility7 effects a similar change in the blue-green region, therefore, 'proportionate comparisons may be made, using the eye to judge unity, that is, using the eye to judge when the two light surfaces 28 andY 29 have the same value. The error due to displacement of the spectra in- 'volves a practically negligible correction when the surfaces 28 and 29 appear equally illuminated when observed through the hood I6 and lens I1. The dial indication is then read at the index 39 and this reading, being in terms of light intensity, can by means of a predetermined factor, be made to signify the proper exposure time for any sensitized photographic paper or any other sensitized media. By means of making a similar reading on the brightest portion of the projected image, the density scale of the negative can be determined and from this set of readings, a paper can be selected which has a iitting exposure scale to match the negative, this exposure scale having been determined for various papers by tests well known to those acquainted with the science of projection printing.

It will be seen that T. have provided a portable instrument for hand-held use including a circular, varying density, light absorbing screen mounted upon the dial, the dial in turn being mounted upon a casing which includes a light source projecting light through the screen upon a comparison block and that the casingls provided with means for viewing the comparison block and comparing the light projected from the lamp within the casing with a beam of light from an outside source.

What is claimed is:

1. A photometer, including a flat circular'casing having a bottom wall and a side Wall and a radially offset housing forming part of the casing but separated from the body of the casing by the side wall, the housing having a top wall provided with a light receiving opening and the side `wall of the casing having an aperture opening from the housing into the casing; a xed light-comparison element disposed within the housing and having a white area receiving light from the iirst-named opening and a second white area joining the first-named area and extending in a planey perpendicular thereto and receiving light from the second-named opening; an electric lamp disposed within the casing and discharging a light beam through the second-named opening onto the second-named white area, a dial mounted for rotation upon the casing and having graduations upon its rim, an index mark on the casing with relation to which the graduations are correlated, alight absorption screen carried by and turnable with the dial and graduated from relative transparency to relativeI opacity and depending into said casing in the path of said light beam, and means carried by the housing for sighting both of said areas at one time to compare the relative illumination thereof.

2. A photometer, including a at circular casing, a dial mounted for rotation upon one side of the casing, an annular absorption screen carried by the dial and rotatable therewith in the casing, a lamp disposed within the casing in the area defined by the screen and projecting a beam of light through the absorption screen, a comparison block outside said area and having two faces at right angles to each other upon one of which faces the light from said lamp impinges, means for admitting light from an outside source along a path parallel to the said one face of said block and onto the other face of the comparison block, and means including a lens having its optical axis coincident with the junction between the two faces of the block whereby the two faces may be 'viewed and comparison between the light received thereon made.

3. A photometer, comprising a fiat circular casing having a bottom wall and a side wall, a

rotary dial constituting a top wall and having l graduations about its rim, an annular band secured along one edge to the inner side of the dial and constituting an absorption screen, a housing extending radially from the casing side wall, the side wall having a light window open- 1 ing from the casing into the housing, the housing having a top wall provided with a light window, a comparison block fixed in the housing and having two perpendicular faces each facing a light window, meanswithin the area dened by 1 said band for directing a light beam through the band and through said rst window toward the block face in line therewith, an index on the casing wall correlated with said graduations, a sight tube opening through the top of said housing and I0 dial and constituting an absorption screen, a 30 housing extending radially from the casing side Wall, the side wall having a light window opening Afrom the casing into the housing, the housing having a top wall providedwith a light win dow, a comparison block fixed in the housing and 88 having two perpendicular faces each facing a light window, means within the area dened by said band for directing a light beam through the band and through said first window toward the block face in line therewith, an index on the casing Wall correlated with said graduations, a sight tube opening through the top of said housing and directed thereinto toward the edge of the block formed at the intersection of said. faces, and a lens in said tube, the optical axis of which intersects said edge, said block being mounted in the housing for adjustable movement toward and away from the tiret-mentioned light window.

5. A photometer, comprising a nat circular casing having a bottom Wall anda side wall, a rotary dial constituting a top wall and having graduations about its rim, an annular band secured along one edge to the inner side of the dial and constituting an absorption screen, a housing extending radially from the casing side wall, the side wall having a light window opening from the casing into the housing,the housing having a top Wall provided with a light Window, a comparison block iixed in the housing and having two perpendicular faces each facing a light window, means within the ara defined by said band for directing a light beam through the band and through said rst window toward the block face in line therewith, an index on the casing Wall correlated With said graduations, a sight tube opening through the top of said housing and directed thereinto toward the edge of the block formed at the intersection of said faces, a lens in said tube, the optical axis of which intersects said edge, and a color lter overlying said lens.

PHILIP R. TARE. 

